PROJECT SUMMARY ABSTRACT Microbicides are products that can be applied to the lining of the vagina or rectum to try and reduce the risk of HIV infection associated with unprotected sexual intercourse. The safety and effectiveness of vaginal microbicides have been studied widely in women at risk of infection but there are very few studies looking at the safety and acceptability of microbicides when used rectally. It is thought that once a vaginal microbicide is developed, it will be used for both vaginal and rectal intercourse. It is therefore important that we look at the safety and acceptability of these products in people who practice rectal sex. In the US, one of the most vulnerable groups for getting HIV infection are young men, especially young Black and Latino men. Our study will be conducted with an ethnically diverse sample of HIV-negative men, 18-30 year-olds, who report engaging in receptive rectal intercourse using condoms inconsistently or not at all. Our ultimate goal is to test whether this highly vulnerable population could safely use the microbicide candidate VivaGel and whether patterns of use of a placebo indistinguishable from VivaGel suggest that the product would be correctly and consistently used in real life circumstances. Our study will be conducted in two stages in which acceptability and adherence will be studied first using a placebo gel applied with a specifically designed rectal delivery device in, or prior to, real-life sexual encounters. Subsequently, the safety of VivaGel will be studied among those men who show the highest adherence to gel use (defined as using the study product during e80% episodes). This safety phase will consist of a single dose of gel followed by one week of daily dosing with the gel. At the beginning of each of these two stages, we will provide all participants condom-use counseling following the Personalized Cognitive Risk-Reduction Counseling protocol, a risk-reduction counseling method to prevent HIV and STIs that showed efficacy in a randomly controlled trial. All of the participants will be closely monitored with clinical, laboratory, and behavioral assessments. Quantitative and qualitative research methods will be used, as well as combination of self-reports, biomarkers, and product recounts. The study will be undertaken by University of Pittsburgh in Pittsburgh, PA., and the HIV Center for Clinical and Behavioral Studies (Columbia University and NYS Psychiatric Institute) in NY, NY. There will be three clinical trial sites: the University of Pittsburgh in Pittsburgh, PA; the Fenway Community Health in Boston, MA; and the University of Puerto Rico Clinical Trial Unit in San Juan, PR. This innovative study will contribute to the development of a safe and effective microbicide for the prevention of HIV infection.